1. Field of the Invention
The present invention relates to a DC-DC converter circuit for converting DC voltage to another DC voltage, a power supply selection circuit for selecting one of a plurality of power supplies, and an apparatus provided with such a DC-DC converter circuit.
2. Description of the Related Art
Many of portable type of electronic apparatuses such as a note personal computer and the like are so arranged that they operate from electric power obtained from a commercial power supply and a battery incorporated therein as well.
Usually, such an apparatus incorporates therein a circuit for changing over as to which source of electric power, the commercial power supply or the battery, is used to operate the apparatus (for example, Japanese Patent Laid Open Gazette Hei.9-182288, and Japanese Patent Laid Open Gazette Hei.9-308102). According to such type of circuit, when electric power obtained from the commercial power supply is supplied to the apparatus, this electric power takes precedence in use, and when the circuit detects that the supply of power from the commercial power supply stops, the supply of power changes to the supply of power from the battery. As another type of the power supply switching circuit, a circuit is arranged in such a manner that, in view of the fact that electric power obtained from the commercial power supply is generally higher in voltage than that from the battery, the supply of power selected is from the electric power of the highest voltage of the plurality of electric powers.
Incidentally, the voltage of a battery decreases as the battery discharges. Thus, an apparatus is provided with a DC-DC converter circuit for maintaining the voltage of electric power used in the apparatus.
FIG. 7 is a circuit diagram showing a first example of a linear regulator. The linear regulator is one type of a DC-DC converter circuit, and it is generally widely used.
A linear regulator section 10 is loaded on an LSI having an input terminal IN through which electric power of input voltage Vin is applied. The linear regulator section 10 converts the electric power of the input voltage Vin to electric power of output voltage Vout (Vin>Vout) lower than the input voltage Vin, and outputs electric power of the output voltage Vout through an output terminal OUT.
Between the input terminal IN and the output terminal OUT, an NPN transistor 11 for output voltage control is disposed, and between the input terminal IN and a base of the NPN transistor 11, a constant current source 12 is disposed. A current outputted from the constant current source 12 flows through the base of the NPN transistor 11 in the form of a base current thereof, and further flows through a collector of an additional NPN transistor 13 in the form of a collector current thereof. An emitter of the NPN transistor 13 is connected to a ground terminal GND, which is grounded. The output voltage Vout of the output terminal OUT is fed to a plus input terminal of a differential amplifier 16 in the form of a potential division by two resistances 14 and 15, while a reference voltage generated by a reference voltage source 17 is fed to a minus input terminal of the differential amplifier 16. An output terminal of the differential amplifier 16 is connected to a base of the NPN transistor 13.
In the event that the output voltage Vout of the output terminal OUT is biased with a voltage higher than a predetermined reference output voltage, the output voltage of the differential amplifier 16 increases, so that a collector current of the NPN transistor 13 increases. That is, of the current outputted from the constant current source 12, one used as the collector current of the NPN transistor 13 increases, and as a result, the base current of the NPN transistor 11 for output voltage control decreases and thereby the output voltage Vout of the output terminal OUT decreases.
Conversely, in the event that the output voltage Vout of the output terminal OUT is biased with a voltage lower than a predetermined reference output voltage, the output voltage of the differential amplifier 16 decreases, so that the collector current of the NPN transistor 13 also decreases. That is, the base current of the NPN transistor 11 increases and thereby the output voltage Vout of the output terminal OUT increases.
In this manner, the electric power of a constant output voltage Vout is outputted from the output terminal OUT.
FIG. 8 is a circuit diagram showing a second example of a linear regulator. The following description sets forth the differences from the first example of the linear regulator shown in FIG. 7, hereinafter.
A linear regulator 10′ shown in FIG. 8 is provided with a PNP transistor 18 for output voltage control, instead of the NPN transistor 11 for output voltage control in the linear regulator 10 shown in FIG. 7. As a result, the output voltage Vout of the output terminal OUT is fed to the minus input terminal of the differential amplifier 16 in form of a potential division by two resistances 14 and 15, while the reference voltage generated by the reference voltage source 17 is fed to the plus input terminal of the differential amplifier 16.
In the event that the output voltage Vout of the output terminal OUT is biased with a voltage higher than a predetermined reference output voltage, the output voltage of the differential amplifier 16 decreases, so that a collector current of the NPN transistor 13 also decreases. That is, of the current outputted from the constant current source 12, one used as the collector current of the NPN transistor 13 decreases, and as a result, the base current of the PNP transistor 18 decreases and thereby the output voltage Vout of the output terminal OUT decreases.
Conversely, in the event that the output voltage Vout of the output terminal OUT is biased with a voltage lower than a predetermined reference output voltage, the output voltage of the differential amplifier 16 increases, so that the collector current of the NPN transistor 13 also increases. That is, the base current of the PNP transistor 18 increases and thereby the output voltage Vout of the output terminal OUT increases.
In this manner, an electric power of a constant output voltage Vout is outputted from the output terminal OUT.
FIG. 9 is a circuit diagram showing a third example of a linear regulator.
A main difference from the second example of the linear regulator shown in FIG. 8 is that the PNP transistor 18 is replaced by P channel MOS transistor 19. With respect to circuit operation, it is the same as that of the second example shown in FIG. 8, and thus a redundant explanation will be omitted.
FIG. 10 is a circuit diagram showing an example of a switching regulator. The switching regulator 20 is also a type of DC-DC converter circuit, and it is generally widely used.
An electric power of voltage Vin is fed through an input terminal IN of the switching regulator, and an electric power of output voltage Vout (here dealing with a step-down type and thus Vin>Vout) is outputted from a second output terminal OUT 2, of first and second output terminals OUT 1 and OUT 2. Between the first and second output terminals OUT 1 and OUT 2, an outside coil 31 is connected. Between the second output terminals OUT 2 and the ground, an outside capacitor 32 is connected.
Elements of the switching regulator 20, except outside coil 31 and outside capacitance 32, are loaded on an LSI.
Between the input terminal IN and the output terminal OUT 1, P channel MOS transistor 21 is disposed. An output of a PWM comparator 26 is connected to a gate of the P channel MOS transistor 21. An output of a differential amplifier 24 and an output of a triangle wave generator 27 are fed to the PWM comparator 26. The PWM comparator 26 will be described later.
The voltage Vout of the second output terminal OUT2 is fed to a minus input terminal of the differential amplifier 24 in form of a potential division by two resistances 22 and 23, while a reference voltage generated by a reference voltage source 25 is fed to a plus input terminal of the differential amplifier 24. Between the first output terminal OUT 1 and a ground terminal GND which is grounded, a diode 28 is connected. A cathode of the diode 28 is connected to the first output terminal OUT 1, and an anode of the diode 28 is connected to the ground terminal GND.
The PWM comparator 26 compares an output voltage of the differential amplifier 24 with a triangle wave signal outputted from the triangle wave generator 27. When the output voltage of the differential amplifier 24 is lower in voltage than the triangle wave signal, the PWM comparator 26 generates a pulse signal of ‘H’ level. When the output voltage of the differential amplifier 24 is higher in voltage than the triangle wave signal, the PWM comparator 26 generates a pulse signal of ‘L’ level. Such a pulse signal is fed to the gate of the MOS transistor 21, so that the MOS transistor 21 turns on or off in accordance with the variation between the ‘H’ level and the ‘L’ level of the pulse signal. That is, the MOS transistor 21 switches the input voltage Vin at the same repetitive frequency as that of the triangle wave signal.
The diode 28, the coil 31 and the capacitor 32 smooth the input voltage Vin after the switching and generate the output voltage Vout.
When the output voltage Vout slightly exceeds a set up voltage, the output voltage of the differential amplifier 24 decreases, so that a pulse width (a pulse width of the ‘L’ level) of the pulse signal generated by the PWM comparator 26 narrows slightly and thereby the output voltage Vout decreases. Conversely, when the output voltage Vout decreases, the output voltage of the differential amplifier 24 increases, so that a pulse width (a pulse width of the ‘L’ level) of the pulse signal generated by the PWM comparator 26 expands and thereby the output voltage Vout increases. Thus, the switching regulator 20 controls the electric power of a constant voltage Vout to be outputted.